Location Based Services (LBS) are a type of services provided to the subscriber based on geographical position. LBS applications include emergency services, navigation, asset tracking, workforce management, location-based events, location-based advertisement, location-based search, and so on. LBS services are expected to grow in the upcoming years. In the US, the wireless E911 service requires operators to report the location of the subscriber making the 911 call with the accuracy of 50 m for 67% of the calls and 150 m for 95% of the calls for handset-based solutions, and 100 m for 67% of the calls and 300 m for 95% of the calls for network-based solutions. The wireless E911 accuracy requirements are usually taken as general accuracy requirements for all types of LBS services. These requirements are mandated by legislation and, at the same time, they are quite stringent to meet the needs of other LBS applications. Worldwide Interoperability for Microwave Access (WiMAX) networks, as well as any other cellular network providing voice services, such as Voice over IP (VoIP) services, need to be compliant with the wireless E911 requirements and be able to provide the location of the user making the 911 call with the specified accuracy. Currently, there are two main technical approaches that may be used to determine the position of the user in a cellular network. The first approach exploits existing global navigation satellite systems (GNSS), for example, the Global Positioning System (GPS) to estimate the position of the user. GNSS-based positioning may be augmented by network assistance, such as Assisted-GNSS or Assisted-GPS. GNSS-based positioning is an effective method, however, it involves installation of a GPS receiver in the communication device, which makes the device more expensive, and furthermore GPS receivers have poor performance in indoor environments where the direct link to a satellite may be blocked. The second approach involves a user having a communication device positioned via the wireless communication network. In this approach, location parameters are extracted from the signal transmitted over the air. Existing communication systems may rely on the following signal processing techniques for user positioning: Angle of Arrival (AOA) estimation, time difference of arrival (TDOA) estimation, time of arrival (TOA) estimation, received signal strength indicator (RSSI) measurements, and so on. A majority of the deployed cellular systems, such as Global System for Mobile Communications (GSM), WiMAX, and/or Long Term Evolution (LTE), uses TDOA-based positioning as a baseline method for user location. This approach is technically simple and effective since it involves synchronization only between base stations of the cellular networks and does not require time synchronization of different mobile stations.
The TDOA method can be implemented in both downlink (D-TDOA) and uplink (U-TDOA). The D-TDOA positioning method measures the difference of time of arrival for signals coming to the positioned mobile station (MS) from multiple base stations (BSs), typically at least three or more. To accomplish such measurements, known training signals, such as preambles or other reference signals (e.g., MIMO-midamble, common pilots or cell-specific reference signals or special positioning reference signals), are transmitted from the BSs to the MS at exactly known time instants. TDOA estimates for different BS pairs are measured and the MS position can be calculated using a trilateration algorithm. From a physical (PHY) layer perspective, the main problem for D-TDOA location is to accurately measure relative time delays (TDOAs) for multiple neighboring BSs in a severe multipath and interference environment. In a deployed communication system, such as a WiMAX network, these measurements may be performed using some training signals. In the IEEE 802.16-2009 and IEEE 802.16m standards (IEEE—Institute for Electrical and Electronics Engineers), preamble signals are considered as an appropriate candidate for performing D-TDOA measurements. In addition in the IEEE 802.16m standard, the MIMO-midamble can be used for the measurements of signal location parameters as well. Both of these signals are different for different sectors and correspondingly BSs of network and are designed to have good cross and auto-correlation properties. Both signals have three orthogonal subsets transmitted on different subcarrier sets that improve cross-correlation properties due to orthogonal transmission. The preamble signals mainly serve for the purpose of frame synchronization and the MIMO-Midamble is mainly designed for the purpose of MIMO channel measurements. Both of these signals are transmitted at the beginning of each frame may have an additional function of being D-TDOA sounding signals. Despite many aforementioned advantages of the preamble physical structure in WiMAX IEEE 802.16m, there are, however, also some limitations associated with their exploitation for the purposes of the D-TDOA positioning. For example, all preamble and MIMO-Midamble signals are transmitted at the same time and are repeated in every frame using the same signal sequence. Hence, in the interference-limited scenario, coherent combining of the received useful signal will also include coherent combining of the same realization of interfering signals, and accumulation of the signals over multiple frames will not allow improving the signal-to-interference ratio (SIR) of the system, but only the signal-to-noise ratio (SNR). Therefore, in such environments, location accuracy of the D-TDOA method using the preambles will be saturated at some level. For typical hexagonal deployment with three-sector BSs, the D-TDOA location accuracy of the IEEE 802.16m system in the case of using standard preamble signals only allocated at the beginning of each frame may not achieve the stringent accuracy requirements of the wireless E911 service because of the interference between different cells. Hence, to improve the accuracy the other training signals have to be used. The transmission of such signals may be coordinated between different BSs to improve severe interference environment that exists during transmission of the preamble or MIMO-midamble signals.
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